goprotobuf: add nigeltao to CONTRIBUTORS (Google CLA).

R=nigeltao, nigeltao_gnome
CC=golang-dev
http://codereview.appspot.com/4436065

CONTRIBUTORS

@@ -10,4 +10,5 @@ Dave Cheney <dave@cheney.net>
 David Symonds <dsymonds@golang.org>
 Ken Thompson <ken@golang.org>
 Mikkel Krautz <mikkel@krautz.dk> <krautz@gmail.com>
+Nigel Tao <nigeltao@golang.org>
 Rob Pike <r@golang.org>